Structural DeSign design issues for structural engineers
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einforced concrete is a construction method that relies on widely understood and historically validated concepts. Traditionally, reinforcing bars are placed in formwork prior to concrete placement. However, many applications require reinforcement to be added to existing structures by means of reinforcing bars grouted into drilled holes, usually with injectable adhesives. This article provides an overview of how reinforcing bars can be designed in accordance with the development and splice requirements of the American Concrete Institute ACI 318, Building Code Requirements for Structural Concrete, using a post-installed adhesive anchor system. Specific reference to the 2012 International Building Code (2012 IBC) and ACI 318-11 will be made because the first International Code Council-Evaluation Service (ICC-ES) Evaluation Service Reports (ESRs) containing provisions for designing post-installed reinforcing bars as “reinforcement” are recognized within the 2012 IBC provisions. Post-installed reinforcing bars, for the purposes of this article, to refer to reinforcing bars installed into hardened concrete using a qualified adhesive anchor system. The primary focus will be to discuss post-installed reinforcing bar design using the development length provisions within ACI 318-11. Alternative design methods for post-installed reinforcement based on anchor theory and shear-friction will also be mentioned.
Post-Installed Reinforcement Design Using Adhesive Anchor Systems By Richard T. Morgan, P.E.
Anchor Design versus Reinforcement Design Richard T. Morgan, P.E., is the Manager for Software and Literature in the Technical Marketing Department of Hilti North America. He is responsible for PROFIS Anchor and PROFIS Rebar software. He can be reached at richard.morgan@hilti.com.
The online version of this article contains detailed reference. Please visit www.STRUCTUREmag.org.
Before discussing the new provisions to design post-installed reinforcing bars for development, it is a good idea to review the current provisions for post-installed anchor design. Post-installed adhesive anchor systems are commonly designed with the Appendix D, Anchoring to Concrete, provisions of the ACI 318 code. Anchor elements used with adhesive anchor systems include threaded rod, internally threaded inserts, proprietary anchor elements and reinforcing bars. ACI 318-11 Appendix D contains provisions for calculating design strengths corresponding to anchor failure modes. Bond strength provisions are given in Part D.5.5, Bond strength of adhesive anchor in tension. The key concept when using Appendix D to design with reinforcing bars is that the bars act as “anchors” (Figure 1). Essentially, when using Appendix D provisions, the bars are designed in the same manner as anchor bolts. This concept assumes the “anchorage” is subject to three possible failure modes in tension: steel failure, concrete breakout and bond failure; and three possible failure modes in shear: steel failure,
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Figure 1. Reinforcing dowels designed using anchor theory.
concrete breakout and concrete pryout. The bars can be treated as a group, such that the effects of spacing and edge distance relative to a specific embedment and characteristic bond strength are considered. Consideration is also given to splitting via the modification factors ψcp,N and ψcp,Na; however, calculation of these parameters is concerned with the increased edge distance required to preclude splitting failure rather than the embedment required to develop the bar to preclude splitting failure. The predictive expressions for concrete breakout, pryout, etc. in Appendix D do not explicitly consider the influence of reinforcement. However, Appendix D does permit consideration of “supplementary reinforcement” or “anchor reinforcement” to enhance the capacity of an anchorage. The term “supplementary reinforcement” in Appendix D refers to reinforcement capable of controlling splitting, or providing an increased calculated concrete breakout capacity. Reinforcement designed for the strength and serviceability of a concrete member should not automatically be assumed to act as supplementary reinforcement for an anchorage. The term “anchor reinforcement” in Appendix D refers to additional reinforcement specifically designed to preclude concrete breakout failure by transferring the loads applied to the anchorage into bars that will be developed. Reference ACI 318-11 D.5.2.9 (tension) and D.6.2.9 (shear) for more information about Appendix D anchor reinforcement provisions. In contrast to development length provisions, Appendix D anchorage provisions are not predicated solely on design controlled by the steel strength of the anchor element. Rather, Appendix D provisions simply provide the means to calculate various strengths corresponding to possible anchor failure modes. Furthermore, Appendix D provisions consider “steel strength” to be defined by the specified ultimate tensile strength (futa)
